Gas plant

ABSTRACT

A gas plant using an Ortloff or similar process comprising the use of a second expander in place of a valve used in most demethanizer reflux systems, wherein a compressor wheel is used to increase the pressure of the partial stream of inlet gas that is used to provide heat to the demethanizer reboilers. This stream remains a separate stream and is used for the flow of gas that feeds the demethanizer reflux expander.

CROSS REFERENCE

This application is based on and claims priority to U.S. Provisional Application No. 62/475,364 filed Mar. 23, 2017.

BACKGROUND OF THE INVENTION Field of the Invention

This invention relates generally to a gas plant and more particularly, but not by way of limitation, to an Ortloff or similar process comprising the use of a second expander in place of a valve used in most demethanizer reflux systems, wherein a compressor wheel is used to increase the pressure of the partial stream of inlet gas that is used to provide heat to the demethanizer reboilers. This stream remains a separate stream and is used for the flow of gas that feeds the demethanizer reflux expander.

Description of the Related Art

Prior to the development of cryogenic gas processing methods, most gas plants used lean oil absorption to remove propane and heavier hydrocarbons from natural gas, with refrigerated lean oil absorption becoming popular in the 1950's. Cryogenic refrigeration (temperatures below −40 deg. F.) and heat expansion engines (turbo-expanders) were used for the removal of nitrogen and helium from gas streams during this time. The first successful application of a turbo-expander for gas processing was a Coastal States plant in Texas, as described in U.S. Pat. Nos. 3,292,380 and 3,292,381. This plant used a single turbo-expander and recovered 30% to 40% of the ethane, as well as propane and heavier hydrocarbons.

An increase in the demand for ethane caused more expander plants to be built, typically using a process similar to the Coastal States design. The gas would enter the plant at a higher pressure and was dehydrated to remove water. The high pressure gas flow would be split and part of the gas would be cooled in a gas to gas exchanger that would also raise the temperature of the cold residue gas so that standard carbon steel could be used. The other portion would provide exchange with fractionation tower liquid to provide heat for vaporization of the tower liquid stream as well as cooling the inlet gas stream. The two streams would recombine and would be further cooled with propane refrigeration if needed. The gas stream would then enter a cold separator that would remove liquid that would damage the turbo-expander. This cold stream would be work expanded by the expander wheel of the turbo-expander. The pressure would be reduced and the stream would be cooled to a temperature lower than −100 deg. F. This stream would enter the fractionation tower that would separate the methane as a gas from the ethane and heavier components that would leave the bottom of the tower as a liquid. The cold methane gas would be reheated in the gas to gas exchanger and recompressed by the compressor wheel of the turbo-expander. It would then be compressed if needed and sold. The ethane and heaver components would be delivered to a pipeline as a liquid stream.

This was the basic design of cryogenic gas plants in the late 60's and 70's with several companies working to improve this method. In particular, Ortloff Corporation developed a number of alternate designs. During the 1980's, the practice of using the Ortloff design with a subcooled gas stream to provide reflux to the top of the tower became a common practice for the cryogenic gas processing industry. While the industry continued to move forward with more patents being issued with improvements to this design, the Ortloff GSP (Gas Subcooled Process) is probably the most commonly used process in the United States. There are several engineering companies that have several more specialized plant designs that use variations of the subcooled reflux process, but the subcooled reflux flow process is the standard gas plant design at this time.

Nonetheless, the current designs may be improved.

Based on the foregoing, it is desirable to provide a process similar to an Ortloff process, but with colder temperature at the top of the tower and better recoveries.

SUMMARY OF THE INVENTION

In general, in a first aspect, the invention relates to a gas plant comprising: a gas inlet, where the gas inlet is capable of dividing inlet gas into a first gas stream and a second gas stream; a first separator and a first expander in fluid communication with the gas inlet such that at least a portion of the first gas stream passes through the first separator and then through the first expander; a second separator and a second expander in fluid communication with the gas inlet such that at least a portion of the second gas stream passes through the second separator and then through the second expander; and a tower in fluid communication with the first expander and in fluid communication with the second expander such that gas exiting the first expander and gas exiting the second expander feeds the tower. The first expander and the second expander may be connected to the gas inlet in such a way that gas passing through the first expander does not pass through the second separator or the second expander and gas passing through the second expander does not pass through the first separator or the first expander.

The first expander may comprise an expansion section, where the first gas stream passes through the expansion section, and a compression section, where the compression section is capable of compressing the first gas stream immediately after the inlet gas is divided into the first gas stream and the second gas stream. Similarly, the second expander may comprise an expansion section, where the second gas stream passes through the expansion section, and a compression section, where the compression section is capable of compressing residue gas exiting the tower.

The gas plant may further comprise a first exchanger, where the first exchanger is in fluid communication with the inlet and the first separator and separately in fluid communication with the tower, such that the first exchanger is capable of cooling the first gas stream prior to entering the first separator using residue gas exiting the tower. The gas plant may further comprise a second exchanger, where the second exchanger is in fluid communication with the inlet and the second separator and separately in fluid communication with the tower, such that the second exchanger is capable of cooling the second gas stream prior to entering the second separator using residue gas exiting the tower. The gas plant may further comprise a third exchanger, where the third exchanger is in fluid communication with the inlet and the first separator and separately in fluid communication with the tower, such that the third exchanger is capable of heating tower liquids using inlet gas.

In a second aspect, the invention relates to a method of processing gas comprising: dividing inlet gas into a first gas stream and a second gas stream; passing the first gas stream through a first separator and then through a first expander; passing the second gas stream through a second separator and then through a second expander; and feeding a tower with the first gas stream after exiting the first expander and with the second gas stream after exiting the second expander. Again, the first expander and the second expander may be connected to the gas inlet in such a way that gas passing through the first expander does not pass through the second separator or the second expander and gas passing through the second expander does not pass through the first separator or the first expander.

The first expander may comprise an expansion section and a compression section, where the first gas stream passes through the expansion section, and the method may further comprise driving the compression section with the expansion section and compressing the first gas stream with the compression section immediately after dividing the inlet gas into the first gas stream and the second gas stream. Likewise, the second expander may comprise an expansion section and a compression section, where the second gas stream passes through the expansion section, and the method may further comprise driving the compression section with the expansion section and compressing residue gas exiting the tower with the compression section.

The method may further comprise cooling the first gas stream prior to passing through the first separator in a first exchanger using residue gas exiting the tower, cooling the second gas stream prior to passing through the second separator in a second exchanger using residue gas exiting the tower, and/or heating tower liquids in a third exchanger using inlet gas.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is s schematic view of the improved gas plant described herein.

Other advantages and features will be apparent from the following description and from the claims.

DETAILED DESCRIPTION OF THE INVENTION

The devices and methods discussed herein are merely illustrative of specific manners in which to make and use this invention and are not to be interpreted as limiting in scope.

While the devices and methods have been described with a certain degree of particularity, it is to be noted that many modifications may be made in the details of the construction and the arrangement of the devices and components without departing from the spirit and scope of this disclosure. It is understood that the devices and methods are not limited to the embodiments set forth herein for purposes of exemplification.

In general, in a first aspect, the invention relates to an improved gas plant design utilizing an Ortloff or similar process comprising the use of a second expander in place of a valve used in most demethanizer reflux systems, wherein a compressor wheel is used to increase the pressure of the partial stream of inlet gas that is used to provide heat to the demethanizer reboilers. This stream remains a separate stream and is used for the flow of gas that feeds the demethanizer reflux expander.

As used herein, the term “inlet gas” may mean a hydrocarbon gas, such as is typically received from an oil and gas well gas gathering system and is substantially composed of methane with the balance being ethane, propane, and heavier hydrocarbons, nitrogen, carbon dioxide, and other trace gases. The term “turbo-expander” and “expander” may mean a work expansion device that consists of an expansion section and a compression section. The expansion section may remove energy by using the pressure drop of a gas to turn an expander wheel. The removal of energy may cause the lower pressure gas to be much colder than it would be if a restriction was used to reduce the pressure. The expander wheel may be connected to the compressor wheel in the compression section, which uses this energy for gas compression.

In the present invention, an inlet gas stream enters at line 10 and is split into two feed streams. One stream may travel through line 11 to valve 15, which is normally fully open. Valve 15 may be used if there is a problem with compressor 20 and/or expander 21. The gas may then travel through line 17 to gas exchanger 12, where it may be cooled by residue gas leaving the tower 90.

The other portion of the inlet gas may pass through line 14 to the compressor section of a turbo-expander 20, which may be driven by the expansion section of turbo-expander 21. This may increase the pressure and temperature of this gas stream. This gas may then be cooled as necessary by exchange with ambient air in cooler 16. The warm gas stream after cooler 16 may also be cooled in a gas to liquid exchanger or any other exchanger that would be needed for other plant uses if the operator wishes to do so. This gas stream may then travel through line 19 to an exchanger or exchangers 30, where the gas may be cooled by providing heat to tower liquids that pass through lines 31, 32, 33, and 34. If the entire gas flow is not needed to heat liquids in exchanger 30, control valve 36 may allow gas to flow from line 35 to line 37.

After traveling through exchanger 30 and/or control valve 36, the gas may flow through line 37 to separator 38, which may allow liquids to fall out and be sent to the tower 90. Gas may leave the top of the separator 38 via line 39 and a portion of the gas stream may travel to exchanger 60.

Line 40 and control valve 41 may be used to divert a portion of the gas from line 39 to line 50, where it may blend with the cool gas leaving exchanger 12. This may allow the inlet gas flow through exchangers 12 and 30 to be adjusted for optimum heat exchange. The gas in line 50 may then travel in the manner used in many cryogenic plants. If additional cooling is needed, the gas may pass through a chiller where a mechanical refrigeration system may use propane or another refrigerant to remove heat from the gas. It may then travel to separator 53 where liquids that might damage the expander 55 may be removed and sent to the tower 90. The gas may exit separator 53 through line 54 to the expansion section of a turbo-expander 55 and then to the tower 90 via line 57. The work extracted from the expansion section 55 may be used to provide energy to a compressor section 56, which may be used to increase the pressure of the gas leaving the process.

The inlet gas from line 39 that enters exchanger 60 may be cooled by heat exchange with cold gas leaving the top of the demethanizer tower 90. The amount of heat exchange can be controlled by bypassing gas around this exchanger through line 61 and control valve 62, if desired. This gas may then flow through line 63, where it may be routed to the top of tower 90 in the conventional manner through line 64 to control valve 65, and then to the tower 90 via line 66 and 67 if the turbo-expander 21 is not in service or if the operator wants to reduce the amount of gas traveling to the turbo-expander 21 for any reason.

When the turbo-expander 21 is in service, the gas flow may be through line 70 to control valve 71, which may be used as a temperature control and/or a pressure control valve. The two-phase stream leaving control valve 71 may enter separator 72, where liquids that might damage the turbo-expander 21 may be removed and sent to the demethanizer tower 90. The gas leaving the top of the separator in line 73 may then travel to the expansion section of the turbo-expander 21. This gas may drive an expander wheel, which may provide power for the compression section 20, as described earlier. The cold, low pressure gas stream may exit the expander 21 via line 74 to line 67 and may enter the top section of the tower 90.

The tower 90 may be a demethanizer/deethanizer designed with trays and/or packing in the manner common to the industry. The condensed ethane and heavier components may fall down the tower as a liquid and may be heated by incoming gas in exchanger 30. The liquid may fall to the bottom of the tower and be removed as a liquid via pump 50 and sent to a gas to liquid exchanger (if used) and then to liquid treating and sales.

The gas stream leaving the top of the tower may be primarily methane with some ethane and small amounts of other gases. This cold stream may pass through line 75 to exchanger 60 where it may be warmed while cooling the feed stream to expander 21. It may then travel to exchanger 12 where it may again be heated while cooling the gas stream that will feed expander 55. From there it may travel through line 76 to compressor section 56, which may be driven by expansion section 55. It may then exit the plant and be sent to gas compression and sales.

The idea of increasing the pressure of the gas stream that feeds the demethanizer reboilers and using this higher-pressure stream of gas for the demethanizer reflux stream may result in a greater pressure drop and temperature drop for the demethanizer reflux stream. This may result in a colder temperature at the top of the tower and better recoveries.

Using an expander wheel instead of a control valve to reduce the pressure of the demethanizer reflux stream may be much more efficient, as it may remove much more heat from this gas stream. This may reduce the heat removal duty of the reflux condenser, allowing a lower temperature residue gas to enter the gas to gas exchanger. This may also lower the temperature of the inlet gas that leaves the gas to gas exchanger. This may reduce or eliminate the need for propane refrigeration, doing away with fuel or electricity costs of the refrigeration compression. If there is no propane refrigeration system, there will be no propane loss, no leak checking, no propane reliefs to flare, reduced flare duty, and lower environmental impact.

The changes to the flow into, around, and out of the reflux condenser as well as the control valve before the scrubber for the demethanizer reflux expander may allow the operator to have more control of the temperature at this point. The scrubber before the expander for the demethanizer reflux stream may be stainless steel, allowing the temperature to be much lower than the traditional cold separator which is often made of carbon steel. This may give a much leaner feed to the top of the tower than designs that use the traditional cold separator for the separator for the demethanizer reflux flow. This may improve the tower profile because the demethanizer reflux stream may have less ethane and heavier hydrocarbons in this stream. This may give it some of the advantages of designs that recycle some of the residue stream back to the demethanizer without the extra compression expense.

Plants may be able to lower the temperature of the gas entering the plant. This may lower the liquid loading of the inlet dehydration as well as allowing a colder gas temperature at the inlet and exit of the gas to gas exchanger. Plants that fractionate the condensate that falls out before the plant and return the ethane and heavier components may be able to remove more liquids by cooling the inlet gas to a lower temperature, which may reduce the cooling work load of the plant process.

The design may eliminate the need for a supplemental heat source to provide enough reboiler duty to maintain demethanizer bottom temperature. This may lower construction and fuel expense. The compressor wheel of the expander may increase the temperature of the inlet gas stream that is providing heat duty to the reboilers, allowing adjustment of the temperature of this stream up to the limits of the aluminum plate fin exchangers to provide reboiler heat.

Using the speed and louver control of the air exchanger before the demethanizer reboilers may allow the operator to control the gas flow ratio between the gas to gas exchanger and the demethanizer reboilers at a constant ratio, which may allow the optimum heat exchange for both exchangers. The current mode of control will divert flow from the gas to gas to the demethanizer reboilers to increase the heat input of the demethanizer reboilers, which can reduce the efficiency of the gas to gas exchanger.

If this process is used in the design of the plant, the additional cooling may allow a higher demethanizer pressure to be used or a lower inlet pressure to be used. This may reduce the amount of compression horsepower needed.

This design can be added to existing plants when inlet gas composition changes and the plant owners need to make a modification to increase recoveries. The expander, separator, gas cooler, and some of the piping may be installed when the plant is in operation and then the plant may have a short shut down for pipeline tie ins before being restarted. The design may be adaptable enough so that this process could be removed from a less economical plant and moved to another plant of comparable size.

Whereas, the devices and methods have been described in relation to the drawings and claims, it should be understood that other and further modifications, apart from those shown or suggested herein, may be made within the spirit and scope of this invention. 

What is claimed is:
 1. A gas plant comprising: a gas inlet, where the gas inlet is capable of dividing inlet gas into a first gas stream and a second gas stream; a first separator and a first expander in fluid communication with the gas inlet such that at least a portion of the first gas stream passes through the first separator and then through the first expander; a second separator and a second expander in fluid communication with the gas inlet such that at least a portion of the second gas stream passes through the second separator and then through the second expander; and a tower in fluid communication with the first expander and in fluid communication with the second expander such that gas exiting the first expander and gas exiting the second expander feeds the tower; where the first expander and the second expander are connected to the gas inlet in such a way that gas passing through the first expander does not pass through the second separator or the second expander and gas passing through the second expander does not pass through the first separator or the first expander.
 2. The gas plant of claim 1 where the first expander comprises: an expansion section, where the first gas stream passes through the expansion section; and a compression section, where the compression section is capable of compressing the first gas stream immediately after the inlet gas is divided into the first gas stream and the second gas stream.
 3. The gas plant of claim 1 where the second expander comprises: an expansion section, where the second gas stream passes through the expansion section; and a compression section, where the compression section is capable of compressing residue gas exiting the tower.
 4. The gas plant of claim 1 further comprising a first exchanger, where the first exchanger is in fluid communication with the inlet and the first separator and separately in fluid communication with the tower, such that the first exchanger is capable of cooling the first gas stream prior to entering the first separator using residue gas exiting the tower.
 5. The gas plant of claim 1 further comprising a second exchanger, where the second exchanger is in fluid communication with the inlet and the second separator and separately in fluid communication with the tower, such that the second exchanger is capable of cooling the second gas stream prior to entering the second separator using residue gas exiting the tower.
 6. The gas plant of claim 1 further comprising a third exchanger, where the third exchanger is in fluid communication with the inlet and the first separator and separately in fluid communication with the tower, such that the third exchanger is capable of heating tower liquids using inlet gas.
 7. A method of processing gas comprising: dividing inlet gas into a first gas stream and a second gas stream; passing the first gas stream through a first separator and then through a first expander; passing the second gas stream through a second separator and then through a second expander; and feeding a tower with the first gas stream after exiting the first expander and with the second gas stream after exiting the second expander; where the first expander and the second expander are connected to the gas inlet in such a way that gas passing through the first expander does not pass through the second separator or the second expander and gas passing through the second expander does not pass through the first separator or the first expander.
 8. The method of claim 7 where the first expander comprises an expansion section and a compression section, where the first gas stream passes through the expansion section, the method further comprising: driving the compression section with the expansion section; and compressing the first gas stream with the compression section immediately after dividing the inlet gas into the first gas stream and the second gas stream.
 9. The method of claim 7 where the second expander comprises an expansion section and a compression section, where the second gas stream passes through the expansion section, the method further comprising: driving the compression section with the expansion section; and compressing residue gas exiting the tower with the compression section.
 10. The method of claim 7 further comprising cooling the first gas stream prior to passing through the first separator in a first exchanger using residue gas exiting the tower.
 11. The method of claim 7 further comprising cooling the second gas stream prior to passing through the second separator in a second exchanger using residue gas exiting the tower.
 12. The method of claim 7 further comprising heating tower liquids in a third exchanger using inlet gas. 